Blood type would be too simple and obvious, but something similar ... you'd need a change-in-humanity for it to work, either an apocalypse that changes human biology, or something like a colony on another planet, where minor differences that never interfered with human mating suddenly are preventing births all over the place. (What happens if you are only fertile with someone whose spinal-fluid pH balance is within a certain range of yours?)

Cue years of scientists trying to figure out what's going on, while people manage to have mates and children at whatever level they do; then you have discovery of The Real Fertility Issue and, what, erasure of current infertile marriages? Not gonna happen. (Especially if those are *maybe* fertile under just the right conditions. Cue bizarre experimentation with diet-based fertility cults.)

If there are "types," they might be known from childhood--or they might not establish until puberty; one's "fertility type" might not be knowable until one is fertile. If it's something as easily testable as blood type, and knowable from childhood, you'd probably eventually get schisms along type lines--Type A people just don't associate with Type B people, and when they fall in love, it's just tragic. Or, conversely, you have Type A brothels that Type O and B people can frequent at will because there's no risk of pregnancy. (Maybe they've fixed diseases; maybe those are dealt with in some other way.) Or upper-class parents arrange "sex friendships" between infertile teenagers, which aren't supposed to affect their later married lives.

The mind boggles. Just knowing that there are people you could & could not get pregnant with (other than "your own sex," which we already have a huge swarm of differences about) would alter social structures; being able to identify them exactly would make drastic changes in legal structures as well.